Dynamic braking system



ff` w 29 1942. H, @GDEN DYNAMIC BRM/ING SYSTEM 2 Sheets-Sheet l Filed Oct., 2.5, i941 www ITB/@mitch l Harold, S. Ogden,

His Attorney Dec. 22, 1942.

H. S.'0GDEN l DYNAMIC BRAKING SYSTEM Filed Oct. l5, 1941 2 Sheets-Sheet 2 MILES PER HUUR IOC BRA K/NG EFFORT Inventor: Harold S. Ogden His Attorney Patented Dec. 22, 1942 UNITED sii-criss PATENT g OFFICE Harold S. Ogden,

Erie, Pa., assignor to General Electric Company, a corporation of New York I Application October 15, 1941, Serial No. 415,060

15 claims. (ol. 17a- 179) My invention relates to dynamic braking systems and particularly to differential dynamic braking systems applicable to gasor Dieselelectric self-propelled vehicles having one or more complete power plants.

It is Well understood that in a self-propelled power plant such as a Diesel-electric locomotive. space is at a premium. Since the traction motors which are generally in use today are essentially constant kilowatt machines over a very wide range of speed, the braking resistors into which they pump power during dynamic braking must have a high ohmic value and a low current capacity at high speed, and acorrespondingly high current capacity and low ohmic value at low speed. Since the kilowtts to be dissipated at the braking resistor are considerable, especially in a locomotive hauling a long train of cars, the resistor itself takes up considerable valuable space on the locomotive. Furthermore, the variable resistor which is generally used for this purpose is uneconomical, for in reducing the rei* q sistance from a maximum value to zero by sequential short-circuiting of portions of the resistor the average amount of resistance used is only half of that provided; i. e., the resistor is only used to approximately one half of its maxibraking resistor 'comprising a plurality of separate resistor sections and switching means for connecting the sections in selectable combinations of series and parallel circuit relation so that various values of total resistance may be obtained while utilizing each section at all times.

I also provide regulating means for maintaining a predetermined gradually increasing field excitation current in the traction motors for each selected connection of braking resistor sections. In order to readjust the tleld current to its proper value for each resistor connection, means are provided for recalibrating the current regulator whenever the are changed. n

My invention itself will be better understood and its objects and advantages further appreciated by referring now to the following detailed braking resistor connections specification taken inconjunction with the acmum capacity. Furthermore, since such a braky ing resistor must be cooled by a forced current of air, it is very desirable to insure that all sections of the resistor are in the braking circuit at all times so that the air passing across the resistor will receive its full share of heat, thereby keeping the Ventilating requirements to a minimum.

Accordingly, it is one object of my invention to provide a new and improved dynamic braking system for a gas or Diesel-electric self-propelled vehicle. l

It is a further object of my4 invention to provide a self-propelled vehicle with a differential dynamic braking system which will supply a smoothly` and gradually increasing braking effort4 having a limited maximum value.

It is a still further object of my invention to provide a dynamic braking system in which no part of the braking resistor is entirely disabled at any time during the braking sequence, thereby to reduce the size and bulk of the resistor required and to conserve Ventilating capacity.

It is another object of my invention to provide i a differential dynamic braking system having neld control means for gradually controlling the braking effort between -points of transfer of the braking resistor connections.

According to my invention, I provide a dynamic companying drawings in which Fig. l is a schematic circuit diagram of an electric vehicle dynamic braking system embodying my invention Fig. 2 is a simpliied circuit diagram of the dynamic braking circuit forming a part of Fig. 1; Figs. 3, 4 and 5 are simpliiied circuit diagrams showing various connections of the dynamic braking resistor; and Fig. 6 is a graphical representation of the braking eiiort characteristic provided by my invention.

Referring now to Fig. 1, I have shown a. multiple power plant Diesel-electric driving arrangement for a self-propelled vehicle comprising a pair of Diesel engines I0 and II connected respectively to drive main direct current generators I2 and I3. The main generator I0 is connected to supply current to a pair of direct current traction motors having armatures Il and I5 and series eld windings IB and I1, respectively. Similarly, the main generator I3 source of control voltage, such as a battery 3|, through a current regulator or voltage controller 32 which will be more fully described hereinafter. In like manner, the main generator I3 is provided with a starting winding 33, a commutating is connected to y supply current to a pair of traction motors havcld winding 34 and a separately excited winding 35 energized by an engine-driven exciter 40. The engine-driven exciter 4I! is supplied with excitation by means of a differential series field winding 4I carrying the line current from the main generator I3 and a separately-excited winding 42 connected to the battery 3i.

Each pair of traction motors is provided with a braking resistor comprising a plurality of separate sections 44, 45, 46 and 41. The pair of sections 44, 45 are permanently connected in parallel with the pair of sections 46, 41. A plurality of switches U, 5|, 52 is arranged to connect the sections 44 and 45 selectively in series or parallel circuit relation. Similarly, a plurality of switches 53, 54 and 55 is arranged to connect the sections 4B and 41 selectively in series or parallel circuit relation.

In operation, the Diesel engines I0 and II are started by using the main generators I2 and I3 as starting motors. This may be done by closing a starting switch GSi for connecting the main generator I2 to the battery 3| and closing a starting switch GSz for connecting the main generator I3 to the battery 3|. The starting circuits for each of the generators are similar and therefore only that for the main generator I2 will be traced by way of illustration. This circuit may be followed from the positive side of the battery 3| through a manually-operable disconnecting switch 56, the contact 51 of the generator starting switch GSi, the generator starting field 25, the commutating eld winding 26, the armature of the main generator I2, the contact 58 of the generator starting switch GSi and a manuallyoperable disconnecting switch 59 to the negative side of the battery 3 I. Similar reference numerals have been applied to the contacts of the generator starting switch GSz. With the Diesel engines I0 and I I operating at their idling speeds, the starting switches GSi and GSz may be opened and the system is prepared for motoring operation.

In motoring operation each pair of traction motors is first connected in series and is then in parallel to one of the main. generators, the circuit for each pair of traction motors being independent. Since the motoring circuits for each pair of motors are similar, only those for the motors connected to the main generator I2 will be traced,

similar reference numerals being applied to cor- 1' responding parts in the other circuit. To begin acceleration, the motors I4, Iii and I5, I1 are con,- nected in series with each other by closing a series switch BD. The motor circuit may now be traced from the positive brush of the main generator I2 through a conductor 6I, a conductor 62, a normally closed braking switch G3, the armature I4, a reversing switch 64 for the series field winding i6, the series eld winding I6 in the direction indicated by the arrow,.the reversing switch B4, the series switch `Ill, the motor armature l5. a reversing switch 65 for the series field winding I1, the series eld winding I1 in the direction indicated by the arrow, the reversing switchIiS, a normally closed braking switch 66, the series eld winding Y29 and the commutating eld winding 2B to the negative brush of the generator I2. The series circuit for the motors i8, 20 and I9, 2| is similar, the reversing switches for the series field windings I9 and 2li being designated as 61 and 68, respectively.

After the vehicle has accelerated to a predetermined point with the traction motors connected in series circuit relation each pair of motors may be transferred to parallel circuit relation in the following manner: Referring to the circuit of the main generator I2, a normally open switch 1U is rst closed to connect a transfer resistor TR in shunt circuit relation with the motor I5, I1. The motor I5, I1 is then temporarily dia-.energized by opening the series switch 6D, the power circuit remaining closed through the motor Y'|4, I6 in series with the transfer' resistor TR. A normally open switch 1I is then closed to connect the motor I5, I1 in parallel circuit relation with the motor I4, I6 and the transfer resistor TR. To complete the transfer of the motors to parallel circuit relation, a normally open switch 12 may be closed to short-circuit the transfer resistor. When the traction motors have reached the limit of their acceleration in the parallel connection, a higher vehicle speed may be attained by partially shunting the series field windings of the motors. For this purpose each traction motor series eld winding I6, I1, 20 and 2| is provided with a shunting resistor 13 and a normally open shunting switch 14.

During motoring operation the regulator 32 is not completely energized and does not function. The inherent voltage characteristic of the exciter 28 is preferably such that during motoring it maintains the desired horsepower characteristic of the electric circuit without external regulation. Such ian-exciter is described and claimed in Patent 1,969,495 issued to J. C. Barry on August 1,

I wish to have it understood that while I have shown a plurality of manually-operable switches for controlling the motor circuit, these switches may be, and preferably are, in practice operated either mechanically or electro-magnetically in proper sequence by means of a main controller of the drum type such as is well known to those skilled in the art. Furthermore, the braking contactors to be described hereinafter may be similarly operated in their proper sequence by the same controller or by a separate braking controller. In order to simplify the circuit connections and to clarify the drawings and description, the controller has not been shown.

In dynamic braking operation one of the main generators, operating at the idling speed of its connected Diesel engine, is used to energize the i'ield windings of all the traction motors through a field excitation circuit including the motor field windings and stabilizing resistors and iii. Due to the very low resistance of the motor series eld windings and the consequent low voltage required for their excitation, the generator develops more than ample Voltage to excite all the fields in series even when driven at the substantially constant low idling speed of the connected engine. The series connection of all the traction motor series iield windings is advantageous in that a larger proportion of the normal generator excitation is required so that the generator operates at a point well up on its saturation curve. Such operation of the generator increases its stability and improves its regulation. The traction motor armatures I4 and I5 are connected in series with each other in a dynamic braking circuit including the associated braking resistor and the stabilizing resistor Bil so that the voltage developed in the traction motor armatures opposes the voltage of the main generator. Similarly, the traction motor armatures I8 and I9 are connected in series with the associated dynamic braking resistor in a second dynamic braking circuit including the stabilizing resistor 8i. Since the stabilizing resistors 3U and 8| are common to the field excitation circuit and the armature circuits, any surges of current in the armature circuits will properly compensate the current in the traction motor field windings.

To establish dynamic braking, the series and parallel motoring switches 60, 10, 1I, 'I2 and 'I4 are opened, the braking switches 63 and 66 are also opened, and a plurality of normally open braking switches 82, 83, 84, 85 and 86 are closed.v

Referring now to Fig. 2, it will be observed that closing of the braking switches 82 to 86, inclusive, completes a closed field excitation circuit which may be traced from the positive brush of the main generator I2 through the switch 82, the traction motor series eld winding I6, the switch 85,the stabilizing resistor 80, the traction motor series field winding I1, the switch 84, the traction motor series eld winding 2U, the stabilizing resistor 8l, the switch 86, the traction motor series eld winding 2l and the switch 83 to the negative side of the generator I2. In this circuit, current flows through the traction motor series field windings I1 and 2| in the same direction as the current flowing through these windings in motoring operation. The current ilows through the traction motor series ileld windings I 6 and 2U, however, in a direction opposite to that flowing through these windings during motoring operation. The effect of this arrangement is to reverse the voltage generated in the armatures I4 and I8 so that the voltages of the armatures lI4 and I5 for one braking circuit and of the armatures I 8 and I9 for the other braking circuit are additive when applied to the braking resistors 44.

The differential dynamic braking connections may now be completed by closing the resistor switches 5I and 54 to complete dynamic braking circuits through the traction motor armatures and the braking resistors. At Fig. 2, the braking resistors have been indicated in simplified form and identied by the reference numeral 44, while Figs. 3, 4 and 5 illustrate the actual connections for the braking resistors, as will be more fully described hereinaiten Referring now more particularly to Fig. 2, it will be observed from an inspection of the polarities indicated at Figs. 1 and 2 that the voltage generated in the armatures I4 and I5 and the armatures I8 and I9 of the traction motors will be additive. Thus the field current and the armature current generated in the armatures i4 and I5 flow in the same direction through the stabilizing resistor 8U, While the iield current and the armature current generated in the armatures I8 and IS flow in the same direction through the stabilizing resistor 8l. Each dynamic braking resistor 44 carries onlyV the arma ture current of the associated traction motor. Due to the differential action of the stabilizing resistors 8 I, the voltage available for exciting the eld windings of the traction motors is dependent over a. wide range of vehicle speeds. It will be understood that where no stabilizing or balancing effects are required, as for the particular instance where no armatures are in parallel, the value of the stabilizing resistors may be zero.

In order to obtain a smooth increase in braking effort as vehicle speed decreases while still providing a limited maximum value of braking effort, I reduce the braking resistance in a plurality of predetermined steps and provide voltage control means in connection with the enginedriven exciter 28 whereby the excitation current through the traction motor field windings is maintained substantially constant for any selected braking resistor resistance value, the voltage controller being recalibrated to maintain a different constant current upon each transfer o! braking resistor connections. As previously stated, each section oi the braking resistor is in use at all times.

Referring now to Figs. 3, 4 and 5, the particular embodiment of the braking resistor shown in l the drawings by way of illustration comprises on the traction motor armature current flowing four sections, 44, 45, 46 and 4l, in combination with six control switches III, 5I, l2, 53, 54 and 55 for connecting the sections in selectable combinations of series and parallel circuit relation. In a particular application it has been found that the three points of resistor control shown in Figs. 3, 4 and 5 are suiilcient to obtain a smooth increase in braking effort as the vehicle speed decreases. To obtain the maximum resistance, the switches 5I and 54 are closed to connect the -pair of sections 44 and 45 in series with each other and to connect the pair 0f sections 46 and 41 in series with each other, the pairs of sections being permanently connected in parallel. If now it is assumed that the re sistance of each resistor section 44, 45 is equal to 3/2R and that the resistance of each resistor section 46 and 4l is equal to 3/4R, the equivalent resistance of the connection shown in Fig. 3 will be'found equal to R. For a second point of control, the switch 50 is closed, the switch 5I is openedfand the switch 52 is closed, in the sequence named, thereby to connect the resistor sections 44 and 45 in parallel with each other, the

connections of the resistor sections 48 and 4I remaining the same. It will now be found that the equivalent resistance of this arrangement, as shown in Fig. 4, is equal to I/2R, the total resistance having been reduced to one half while still utilizing all the resistor sections. Similarly, to obtain a third point of control in which the equivalent resistance is equal to I/4R, the parallel connection of the sections 44 and 45 is left unchanged while the sections' 46 and 41 are connected in parallel with each other by iirst closing switch 53, then opening the switch 54 and nally closing the switch 55. Attention is directed to the fact that the order of switch operation is important in order to maintain the circuit closed at all times.

Referring now to Fig. 6, a braking eiort curve similar to the curves IUI to H5, inclusive,

will be obtained for each selected connection ci' the braking resistor under the condition that the current generated by the main generator I2 remains at a constant value. In order to obl tain a smooth transfer from one such braking effort curve to another, thereby to obtain a resultant braking effort which increases gradually as the vehicle speed decreases, the regulator 32 is automatically recalibrated upon each change of braking resistor connections and the traction motor field current is allowed to gradually build up to a new constant value. For this purpose the winding 30 of the engine-driven exciter 28 is connected across the battery 3l in series with a variable control resistor 90 forming part of the iield voltage controller 32. As shown, the controller 32 comprises a movable conducting segment 9| for gradually short circuiting the control resistor 90 in response to the energization of a xed actuating coil 92 and a noating coil 93. The xed coil 92 is connected directly across the battery 3l 'and carries a predetermined current. The iioating coil 93 receives its energization from a shunt 34 in the circuit of the main generator I2. It will be evident that, for the dynamic braking connection, the shunt 94 lies in the traction motor field exciting circuit. Thus thecoil 93 carries a current which is proportional to the traction motor field exciting current and actuates the conducting segment 3| in such a manner as to tend to maintain this current constant.

In operation, when the braking resistor sections are connected as shown at Fig. 3 and the regulator 32 has its initial setting, the braking v- When decreasage required by the resistor. Thus, the resultant braking effort can build up through the ,curves |01 to H0 as the traction motor field current is progressively reset at new constant values. Between the curves H and lll -the braking resistor connections are changed to those shown at Fig, and the regulator'BZ is again reset so that the braking eiIorts can hel made to progress through the curves III to H5 by varying the resistor 95. The resultant continuous operating point must fall below the curveA of Fig. 6, since this curve represents the limit of operation of the curves 4IM to H5, inclusive, as determined by the capacity of the system. The above operating sequence will ordinarily be controlled from a manually-operable controller arranged to carry out the proper braking resistor switching operations and simultaneously to recalibrate the regulator 32 as well as to provide intermediate control notches for selectably resetting the resistor 95 between changes of braking resistor connections.

I wish to have it understood that it is not proper sequence by means of a rotatable manually-operable controller ofthe drum type such as is well known to those skilled in the art.

From the foregoing detailed description, it will now be evident that I have provided a dynamic braking arrangement in which the braking resistor comprises `a plurality of sections selectably connectable in various circuit relationships such that the over-all resistance may be changed in a plurality of steps, Awhile making use of every section at all times. Furthermore, I have provided a differential dynamic braking system utilizing a resistor of this type and in which a smooth and gradual increase in braking effort is obtained as the vehicle speed decreases by maintaining constant the traction motor neld exciting current at various values during each interval between transfer of braking resistor connections.

While I have illustrated one preferred embodiment of my invention by way of illustration, many further modifications Will undoubtedly occur to those skilled in the art, and I therefore Wish to have it understood that I intend by the appended claims to cover all such modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States, ls:

1. In a dynamic braking system, a momentumdriven dynamo-electric machine having an armature and a series eld winding, a braking resistor connected to complete a dynamic braking circuit including said amature, voltage generating means connected in series with said eld winding across a portion of said dynamic braking circuit thereby to supply an excitation current to said series ileld winding, regulating means operable in conjunction with said voltage generating means to maintain said excitation current substantially constant, and means for changing the resistance of said braking resistor in a plurality of predetermined steps and simultaneously recalibrating said regulating means.

2. In a dynamic braking system, a momentumdriven dynamo-electric machine having an armature and a series field winding, a multi-section braking resistor connected to complete a dynamic braking circuit including said armature, a subsistor in selectable combinations of series and parallelcircuit relation thereby to change the equivalent resistance of said resistor in a pluessential to the practice of my invention that thefregulator 32 be responsive solely to motor field excitation current, but that, if desired, the regulator may be responsive in part to the motor amature current.

As previously mentioned, it is necessary to recalibrate the current regulator 32, whenever 'the resistance of the dynamic braking resistor is changed. For this purpose a variable calibrating resistor 95 is connected in series with the floating coil 93. While I have shown the calibrating resistor 95 and the various braking resistor control switches to 55, inclusive, as manually-operable, I wish to have it understood that these switches and the calibrating resistor may be, and preferably are, controlled either mechanically or electromagnetically in their rality of steps, and field control means for said generator responsive to said excitation current vand arranged to maintain said current substantially constant at predetermined selectable values while said resistor sections remain connected in any selected one oi' said combinations.

3. In a dynamic braking system, a direct current momentum-driven dynamo-electric machine having an armature and aseries field winding, a multisection braking resistor connected to complete a dynamic braking circuit including said armature, a direct current generator for supplying an excitation current to said series field winding, switching means arranged to connect said generator and said series field winding to a portion of said dynamic braking circuit in such manner that the voltage of said generator opposes the voltage developed in said armature during dyasoaiss namic braking, second switching means for connecting said sections of said braking resistor in selectable combinations of series and parallel circuit relation thereby to change the equivalent resistance of said braking resistor in a plurality of steps, and eld control means operable in conjunction with said generator to maintain said excitation current at predetermined selectable substantially constant values while any selected connection is maintained by said second switching means. i

4. In a dynamic braking system for a self-propelled electric vehicle, a plurality of direct current traction motors each having an armature and a series iield winding, a plurality of multisection dynamic braking resistors for said motors, iirst switching means arranged to complete a plurality of dynamic braking circuits each including at least one of said armatures and one of said braking resistors, a substantially constant speed direct current generator for supplying excitation current to said series eld windings, second switching means arranged to connect all said series held windings in an excitation circuit in cluding said generator and a portion of each of said dynamic braking circuits, third switching means for connecting said sections of said dynamic braking resistors in selectable combinations oi series and parallel circuit relation thereby to change the equivalent resistance of said resistors in a plurality of steps, eld control means operable in conjunction with. said direct current generator to regulate said excitation current while said equivalent resistance remains at any selected value, and means for resetting said eld control means upon operation of said third switching means.

5. In a dynamic braking system for a selipropelled electric vehicle, a plurality of pairs of direct current traction motors each provided with an armature and a series field a direct current generator for supplying current to said motors to drive said vehicle, an internal conibustion engine for driving said generator, a multisection dynamic braking resistor for each said pair oi' motors, first switching means arranged to complete a plurality oi dynamic braking circuits, each said circuit including one of said dynamic braking resistors and the armar-tires o! at least one pair or" said direct current motors, a stabilizing resistor connected in each of said braking circuits, second. switching means operable when said internal combustion engine is rotating at its idling speed to connect said generator to a ield excitation circuit including all oi said series iield windings and said stabilizing resistors, said dynamic braking circuits and said ileld excitation circuit being so interconnected by means of said stabilizing resistors that the voltage of said generator opposes the voltages developed in said armatures during dynamic braking, third switching means for connecting the sections ci said inu ti-section dynamic braking resistors in selectable combinations or" series and parallel circuit relation thereby to change the equivalent resistance of said resistors in a plurality oi steps, voltage control means operable in conjunction y with said generator to regulate the current in said series iieid windings while said equivalent rcsistance remains at any selected value, and means for resetting said voltage control means upon operation of said third switching means.

6. In a dynamic braking system, a momentumdriven dynamo-electric machine having an armature and a series neld winding, a braking resistor 'ssii connected tc complete a dynamic braking circuit including said armature, voltage generating means connected in series with said iield winding across a portion of said dynamic braking circuit thereby to supply an excitation current to said series field winding, switching means i or changing the resistance of said braking resistor in a plurality of steps, voltage control means operable in conjunction with said voltage supply means to maintain said excitation current substantially constant at selectable values while said resistance remains at any selected value, and means operable in conjunction with said switching means for changing the calibration to said Voltage control means.

7. In a dynamic braking system, a momentumdriven dynamo-electric machine having an armature and a series iield winding, a braking resistor connected to complete a dynamic braking circuit including said armature, a substantially constant speed generator connected in series with said series ield winding across a portion of said dynamic braking circuit thereby to supply to said series Winding an excitation current, a regulator operable in conjunction with said generator to maintain said excitation current substantially constant, means for reducing the resistance of said braking resistor in a plurality of predetermined steps, voltage control means for said regulator responsive to said excitation current, and means for changing the calibration of said voltage control means thereby to select a predetermined excitation characteristic to be maintained for each predetermined value of said resistance.

8. in a dynamic braking system for a self-propelled electric vehicle, a direct current traction motor provided with an armature and a series field winding, a direct current generator, an internal combustion engine for driving said generator, an exciter for said generator driven by said internal combustion engine, an exciting winding for said exciter, means for energizing said exciting winding, a braking resistor connected to complete a dynamic braking circuit including said armatine, switching means for connecting said generator to supply excitation current to said series nelol winding, andineans responsive solely to said excitation current to control the energizetion of said exciting windingt 9. In a dynamic braking system for a self-propelled electric vehicle, a direct current traction motor provided with an armature and a series eld winding, a direct current generator, an intermal combustion engine connected to drive said generator, an exciter for said generator driven by said internal combustion engine, an exciting winding for said exciter, a resistor connected in series circuit relation with said exciting winding, a dynamic braking resistor connected to complet a dynamic braking circuit including said armature, switching means for connecting said generator in series circuit relation with said series field if r across a portion oi' said dynamic braking circuit thereby to supply an excitation current to said series eld winding, and electromagnetic means responsive to said excitation current arranged to control said resistor thereby to maintain said excitation current substantially constant.

l0. ln a dynamic braking system for a selfpropelled electric vehicle, a direct current traction motor provided with an armature and a series field winding, a direct current generator, an internal combustion engine connected 'to drive said generator, an exciter for said generator driven by said internal combustion engine, an exciting winding for said exciter, a control resistor connected in series circuit relation with said exciter winding, a dynamic braking resistor connected to complete a dynamic braking circuit including said armature, switching means for varying the resistance of said dynamic braking resistor, second switching means for connecting said generator in series circuit relation with said series iield winding across a portion of said dynamic braking circuit thereby to supply an excitation current to said series eld winding, means responsive to said excitation current for varying said resistance of said control resistor to maintain said excitation substantially constant, and means operable in conjunction with said switching means for varying the response of said resistance varying means.

11. In a dynamic braking system for a selfpropclled electric vehicle, a plurality of direct current traction motors each having an armature and a series eld winding, a direct current generator, an internal combustion engine for driving said generator, an exciter for said generator driven by said internal combustion engine, an exciting winding for said exciter, a control resistor connected in series circuit relation with said exciting winding, a plurality of multi-section dynamic braking resistors for said motors, first switching means arranged to complete a plurality of dynamic braking circuits each including at least one of said armatures and one of said braking resistors, second switching means arranged to complete an excitation circuit for said series iield windings including said generator and a portion of each of said dynamic braking circuits, said generator supplying an excitation current to said series field windings, third switching means for connecting said sections of said dynamic braking resistors in selectable combinations of series and parallel circuit relation thereby to change the equivalent resistance of said resistors in a plu rality of steps, an electromagnetic relay having an actuating coil responsive to said excitation current arranged to vary the resistance of said control resistor, a Calibrating resistor in series with said actuating coil, and means operable in conjunction with said third switching means for varying the resistance of said calibrating resistor.

12. ln a dynamic braking system ior an electric motor, the combination of a motor having field winding, a multi-section brain resistor for said motor, and switching :i cennecting the sections of said braking resistor selectable combinations of series and lparallel circuit reiation to reduce the equivalent resistance of said resistor without disabling any of said sections.

i3. In a dynamic braking system, a momentum-driven dynamo-electric machine having an armature, a dynamic braking resistor including a plurality" of sections connected to complete a dynamic braking circuit including said armature, means for connecting at least two pairs of said sections permanently in parallel circuit relation, and switching means for connecting the individual sections of each oi said pairs selectively in series or parallel circuit relation.

14. in a dynamic braking system, a momentum-driven dynamo-electric machine having an armature and a series field winding, a dynamic braking resistor including a plurality of separate sections arranged to be connected to complete a dynamic braking circuit including said armature, voltage generating means connected in series with said series field winding across a portion of said dynamic braking circuit thereby to supply an excitation current to said series field winding, switching means for connecting at least two pairs of said separate resistor sections permanently in parallel circuit relation, second switching means for connecting the separate sections of each oi said pairs selectively in series or parallel circuit relation, and means responsive to said excitation current arranged to control said voltage supply means to maintain said excitation current substantially constant.

l5. in a dynamic braking system, a momentum-driven dynamo-electric machine having an armature and a series eld winding, a dynamic braking resistor including a plurality of separate sections of equal resistance arranged to be connected to complete a dynamic braking circuit including said armature, Voltage generating means connected in series with said series field winding across a portion of said dynamic braking circuit thereby to supply an excitation current to said series field winding, switching means for connecting at least two pairs of said separate resistor sections permanently in parallel circuit re lation, each of said pairs comprising two sections of equal resistance and each section of one oi said pairs having double the resistance oi' each section of said other pair, second switching means for connecting the separate sections of each of said pairs selectably in series or parallel circuit relation, and means responsive to said excitation current arranged to control said voltage supply means.

HAROLD S. GGDEN. 

